Here's the response from the neighbor building the deck...
The 3/8" steel suspension cable is a good deal higher than 10 feet above
the deck at the ends. Currently that 3/8" steel cable is anchored at a
tree about 25 feet above the deck at the beginning end of the deck.
Also, the 100 foot final length of the deck was a guess that is probably
a bit high, where perhaps 80 feet might be closer to the final length. So
the 11 degrees may no longer still be the result of the calculations.
Looking at the photos, the angle of the cable looks like around 30
degrees to me. If the height of the suspension cable above the deck is 25
feet, and the length of the deck is 80 feet, we now have 32 degrees.
Using his tension formula, we get a tension of 4,718 pounds.
So, I believe, that means the 3/8" suspension cable can support almost
six times what we are assuming.
While the deck was originally supposed to be free floating, since we
decided to anchor the close end of the deck on the dirt path (so that
people could just step onto the deck from the path), that end of the deck
is now supported by the two fence posts, so half of its weight goes away.
If, additionally, we add another fence post, in a "T" shape support, at
the end of the first 16-foot-long section, then the weight of that first
16-foot-long section goes away completely, as does half the weight of the
next 16-foot-long section. And we still have the option of supporting the
other half by attaching it to the small redwood trees, along with half of
the third section.
If we really did get to 28,000 pounds of tension, the trees would pull
closer together, reducing the distance, and making the angle steeper. It
thus gets asymptotically harder to actually put that much tension on the
The treehouse itself, when it's built, will be supported mostly by the
Some validity to the above but...
While it is true that the end that is resting on the ground does have
that support, it's not necessarily so that the remaining load on the
cable is only half; it depends on the actual geometry of the
configuration. It _might_ be half; could be more, could even be less.
The previous "analysis" was simply intended as a demonstration of the
effect geometry has on the overall tension required in the cable
(actually, any supporting member--that it is cable in that sense is a
distraction) is that which will, for the given arrangement, provide a
vertical component that balances the gravity forces plus applied loads.
The key lesson intended to be shown is that the vertical component is
dependent upon the angle of the tension force.
Thinking that "it's harder to put that much tension on the cable" isn't
really so--while the trees may move some, the actual counteracting force
is whatever it needs to be given the loading. If the distance is
shortened enough, yes, the angle will increase but that's going to
exacerbate the other issues raised of whether the root structure can
sustain that continued side load (increased moment at the base)
indefinitely, particularly with the addition of wind load and ground
saturation and all the other things that go on.
I do agree that with the one end supported that certainly helps and the
idea of the second ground support at the outer end is also a good one
but I'd still think it only prudent to get the input of a competent
engineer to evaluate the overall structure, including the questions of
the ability of the soil to hold and what would be realistic wind loads
and so on. While one would presume nobody would be silly enough to be
out there during a storm, it seems a sizable investment in both time and
money to risk losing it all the first thunderstorm or the next coastal
front with a good blow. It's certainly not unheard of uncommon for them
to have 90 mph in WA where daughter is; don't know how far this is from
coast nor elevation and such but would think it not unreasonable
It's just imprudent imo to not have more than just a "seat o' the pants"
look at something this ambitious and that has such a potential for
serious consequences if these guesstimates are wrong. Given the $$ that
have and are obviously going to be expended, a few more thrown at the
design/safety issues would seem a good investment. (Or, is the issue
that if does that it'll get stopped because it isn't within whatever
covenants are in place for the location?)
Anyways, I've had my say; simply hoping nothing does go wrong but it
seems a very risky venture as is...
(*) The unique thing about cable is that unless the loading is such as
was presumed before that the cable is essentially straightened by the
load, the tension is variable along the cable
Arfa Daily wrote, on Tue, 28 Oct 2014 02:05:05 +0000:
That's a good suggestion, as we have tools all over the place!
Usually, we tie in when we're out on the cable, with no firm footing:
And, we tie in when we're working on the steep slope below the deck:
The harnesses we use are the same ones we use for climbing the hills:
Here's my setup, for example, as I was digging the fence post holes:
And, here's what I used when I had to double-line rappel downslope:
Even so, we've had a few of the typical gotchas, from dropping the
spool of wire (where it rolled a hundred or so feet downhill before
getting wedge under a fallen tree) to the inevitable search for missing
glasses and hammers.
Such is the nature of working in the trees...
All the supporting cables are above the deck.
It's a bit hard to see in that picture from below today, but the 3/8"
steel cable is glinting in the sunlight a few feet *above* the deck.
The deck is not attached to *anything* other than the cable on the free-
floating end, as we build it out. In fact, while it's hard to tell from
the angle of this picture, but the end of the second 16-foot-long section
is still shy of the leftmost redwood by about a foot, and maybe it's two
feet shy of the right-most redwood tree:
Here is a picture taken earlier in the day, before we planked the second
16-foot-long section, showing how the 10-foot-wide deck is suspended from
the steel cables (this is the second of the two 16-foot-long sections):
This is a good point, in that sometimes it pours out here, and this is a
steep slope, so the runoff could be great.
We just had a half inch of rain over the weekend, and this is a shot of
the cables and the people on the decking (the dog won't go on the deck):
We're still about a foot from the left-most redwood here, and about two
feet from the right-most redwood, so we "can" attach to them (if decided)
with the third 16-foot deck section:
Thanks for all the insight.
Below is the owner's response to your concerns.
BTW, I created an animated GIF of the entire process, as I see it,
but I can't get Flickr to show the animations since Flickr turns
an animated GIF into a static JPG.
I'll post the animation separately, if I can figure out how to
preserve the animation, but here is the starting point static JPG:
Here is where we are right now:
And here is the penultimate ending point static JPG:
Here's the owners response to your valid concerns ...
I wonder if they realize how huge the final redwood tree is?
The tree probably weighs in excess of 2,000 tons, and has a 30 foot
The smaller set of redwood trees I would estimate weighs 15 tons.
In a wind of 50 mph, the small tree experiences 200,000 pounds of force
due to the wind.
The idea that 28,000 pounds of tension on a cable is more than it
encounters in a light wind does not seem tenable.
The root structures of both trees routinely handle much larger forces
during a typical day.
A wind blowing at 100 miles per hour generates 25 pounds of force per
If that wind were blowing straight down on 800 square feet of deck, we'd
have 20,000 pounds of force.
I consider that unlikely. :-)
Edge-on, we have 67 square feet, or 1,666 pounds of force. But that is
also somewhat unlikely.
Sideways forces will add a little to the cable tension, but will mostly
be taken up pushing against the trees and the support posts.
The deck will weigh in the neighborhood of 5,000 pounds, and has 800
square feet of maximum surface area. Lifting that, requires 6.25 pounds
per square foot, or a wind speed straight up of 50 miles per hour. But
the deck is held down at the ends and in the middle by either trees or
posts, which also limit the amount it can tilt or twist. The surrounding
trees limit the wind considerably.
The assumption that the engineering is "seat of the pants", or that the
mathematics have not been done is incorrect, but the ideas are all good
because I don't want to miss something, by not thinking about it at least.
Let them know that I appreciate their advice!
(Please invite them to lunch on Wednesdays in Redwood City if they're
The ones with the longer threads were really stinky because
you needed four hands, while suspended on the cable, to
screw them in.
The ones with the shorter threads only take 3 hands.
Here's an animated GIF, I just made, of the suspension bridge...
Danny D. wrote, on Tue, 28 Oct 2014 20:48:36 +0000:
Flickr turned the animated GIF into a JPG but tinypic seems to keep
it as an animated GIF ...
Here's my rendition, as I understand the plans so far ...
I am omitting the actual building structure, but what you see here
are the following:
1. The starting point, on a steep slope, with a path near the top:
2. All brown lines are 16-foot long lengths of lumber:
3. This approximates the "ladder network" you've seen in the photos:
4. This was the first (thin) cable that went from tree to tree:
5. From that thin cable, we hung two large safety cargo nets:
6. Then we hung the thick cable, which was initially 250 feet long:
7. We sunk two fenceposts, so that the platform rested on the ground:
8. Then we built & suspended the first 16-foot by 10-foot section:
9. Yesterday, we hung the second 16-foot-long section which is a
foot or two shy of the smaller redwood pair of trees:
10. The plan is to add successive 16-foot sections, one by one:
11. We keep that up until we finally reach the big redwood tree:
12. And, finally, we'll add 8'x4' sheets of plywood as a railing:
After that, we begin to build the actual treehouse, complete with
WiFi, refrigerator, bar, running water, and heating (no kidding).
It will take time, of course, so, I'm not sure if I should continue
to update this thread, but, since we've never done this before, any
and all advice is welcome.
PS: Jeff Liebermann and SMS are both welcome to attend the Internet
WiFI setup party since they both live in the area!
There's almost no way you would have known how absolutely huge the
big redwood tree is downslope.
There were a few pictures of it, early on, in a different thread,
when we had laid the first few ladders and the cargo netting.
As you can tell from this diagram, there are three sets of trees
that matter, for our purposes (although many other trees exist):
Here's a quick look downhill, from the path near the uphill path:
This is the uphill anchor point, on a small Monterey Pine tree:
Here's a picture of the two small redwoods at the 1/3 point:
Here's another picture of those small redwood trees where you
can see we strung a cargo net across so we could get to the big
This cargo net is how we get over to the big redwood which is
about 30 or 40 feet downslope of the beginning of the netting:
But, I don't seem to have a picture of the big redwood for you,
so, I'll need to take one and upload it so you can see how massive
it is. It's about 100 feet downslope, and it's hard to get back
up that slope, so, I don't generally go down there unless I need
to. But, I'll do that for you, especially since you've been so
helpful with the engineering advice.
I don't need a picture of the big redwood; I'm perfectly willing to
allow as how they can and do get big; I've been through redwood country
a number of times. I was simply noting from the pictures posted near
that point in the thread there didn't seem to be much of any real size
and was more concerned of potential on the root system with the load
than whether the tree itself was sufficient presuming it was.
When the response to the question regarding angles for trying to
estimate tension needed to provide a given vertical force component
includes the justification that the angle will increase owing to the
tree flexure doesn't lend itself to thinking they're terribly big,
either. And, again, just "devil's advocate" position raising the
From a diagram such as that with a few measurements one could get at
least a reasonable approximation using simple-enough analyses as
outlined in the following (beginning at 7-30ff)--
I drew it, by hand, with Kolourpaint, on Linux, which,
according to Wikipedia, is a Microsoft-Paint like drawing app:
After drawing each line, I just saved the file to a new name,
e.g., drawing01.jpg, drawing02.jpg, drawing03.jpg, etc.
Then, I slapped it all together with this Linux command:
$ convert -delay 50 -loop 0 *.jpg animatedplans.gif
That created this:
The "loop 0" just means loop forever; and the delay is something
like 50 milliseconds between images, I think.
I use this method only because it's trivially simple to do,
so, I'm sure there are *better* ways to make animated drawings.
For example, on Windows, I'd just draw using "Paint.NET" or even
Microsoft Paint. Then, I'd slap it together using one of the
programs described here:
How to make animated GIFs, by PC Magazine
Danny D. wrote, on Wed, 29 Oct 2014 05:04:53 +0000:
I found only one picture of the big redwood, but it doesn't show
how massive the trunk is (measured at 30 feet in circumference).
There's actually a person, close to the tree trunk up there, in
the cargo net, setting up the blocks of wood for the cable to go
around (this picture was taken a few weeks ago).
Here are a few things that come to mind:
Has the owner checked with his insurance provider to see if he is protected
from liability? Things like this are known as an attractive nuisance and
everyone involved might be at risk should anyone get hurt.
Have you considered corrosion of the cable? Is it steel, galvanized, or
Do you have an inspection plan in effect to detect future failure
You might apply some paint to the cable clamps to serve as a witness mark to
see if anything slips.
It is pretty neat and will have all the kids in the area interested.
josephkk wrote, on Wed, 29 Oct 2014 21:11:45 -0700:
I would tend to agree, as the big redwood is massive (30 feet in
For scale, there's actually a person, wearing blue, in the cargo net
right next to the tree, fixing the blocks for the cable that we later
wrapped around that tree.
BTW, even the little redwoods are not all that little:
Tom Miller wrote, on Wed, 29 Oct 2014 15:56:20 -0400:
The owner is an ex Google executive, so, he has the disposition to have
lots (and lots) of "attractive nuisances" on his property! For example,
you can travel in another part of his property, high up, from tree to
tree to tree to tree (etc) by cargo net, for HUNDREDS of feet!
I always find a way to take my grandkids to his place for fun stuff.
Steel. When I asked, he said there's plenty of zinc fittings, so, he
wasn't worried about rust.
Good question. I'll ask.
This is a GREAT idea!
I will suggest that to the owner!
Kids love his place. I can't count the number of "attractive nuisances"
he has on his rather large (scores of acres) property.
I apologize that most of my pictures were from the uphill side (where
the suspension bridge is currently forming), where those trees are puny
in comparison to "General Sherman" (which is what we call the big one).
The only time I climbed down the 100 feet to General Sherman was when we
were setting up the cables around it, and I was the gaffer who passed up
tools and supplies.
So my only pictures of General Sherman are the ones I showed, which don't
quite show the massive girth of the thing, especially at the bottom,
because what you see above is already split in two.
I agree with you, that when I first saw the angle stuff, I too wondered
about bending a tree that much to make *any* difference. I'll forward
your comments above on to the owner to see what he makes of that.
I will forward that "Chapter 7: Forces in Beams and Cables" PDF to the
owner, who, while he isn't an engineer, he has multiple graduate degrees
and can handle almost anything we throw at him (he was an early Google
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